Surface acoustic wave (SAW) filters typically comprise interdigital transducer elements deposited on a piezoelectric substrate. SAW filters are widely used in various telecommunication systems due to small size, low loss characteristics of the implemented SAW resonators. SAW filter performance is generally dependent upon the features of the piezoelectric substrate. Low loss, smooth broadband, good rejection and sharp shape factor filter characteristics may be achieved by providing a piezoelectric substrate which exhibits high coupling coefficient, temperature compensation, high resonant Q factor and strong suppression of plate modes. Lithium Tantalate (LT) and Lithium Niobate (LN) substrates exhibit a high coupling coefficient. Rotated Y-cuts of LT exhibit coupling coefficient in the range of 8% to 10% and have been described by Ueda et al. in U.S. Pat. No. 6,037,847 and by Naumenko et al. in U.S. Pat. No. 6,556,104, the disclosures of which are herein incorporated by reference in their entirety. The coupling coefficient of LN has been shown to exhibit a coupling coefficient as high as 23%. However, these high coupling piezoelectric substrates typically exhibit an undesirably significant temperature drift.
Two approaches have been investigated to reduce the temperature drift of the high coupling substrates by bonding the high temperature coefficient expansion piezoelectric substrate to a low temperature coefficient expansion substrate. Taguchi et al. in U.S. Pat. No. 5,998,907, the disclosure of which is herein incorporated by reference in its entirety disclose a number of bonded SAW device structures for purpose of temperature compensation. Taguchi discloses combining two piezoelectric layers with one layer bonded directly to a second layer having a lower temperature coefficient of expansion (TCE). In another embodiment disclosed by Taguchi comprises of a thin film insulation layer of silicon dioxide of 1000 Angstrom joined directly with the piezoelectric substrate and a TCE carrier of Si for purpose of temperature compensation. While attempts to control temperature characteristics are sought, structures as proposed above lead to spurious and undesirable responses due to a direct capacitance coupling between the electrode pattern and carrier, typically a Silicon material. This results in a poor filter performance characteristics.
Abbott et al in U.S. Pat. No. 7,105,980 discloses a SAW composite device comprising a piezoelectric substrate, a surrogate carrier and a bonding film securing the piezoelectric substrate to the carrier wherein the bonding film is a silicon oxide layer with at least one micron thickness to reduce capacitance coupling.
Other known embodiments include bonded wafer SAW filters in which the piezoelectric substrate is bonded with an adhesive layer to a lower temperature coefficient expansion carrier wafer. The TCE material used may be Si or alumina. Unfortunately, and as will be detailed late in this specification, such device structures produce plate mode resonances that distort the response of the resonators. There is a need to overcome this disadvantage.